Single ended sensing is often used with a matchline (ML) in Content Addressable Memory (CAM) and a bitline in eight-transistor Static Random-Access Memory (8T SRAM). Often, these lines are held low when idle to save leakage power. To perform an operation, the matchlines or bitlines are precharged and then evaluated (e.g., sensed).
As technology scales to submicron geometries, random device variation (RDV) is becoming more prominent. RDV of parameters such as transistor length, transistor width and transistor threshold voltage could be significant even in identically designed neighboring devices. The effects of RDV are especially evident in the design of semiconductor memories. Because most memories rely on sense amplifiers to detect small voltage signals on largely capacitive array lines, RDV in the memory cells as well as sense-amplifier devices can produce incorrect results. To improve reliability, memory designers tune their sensing circuits conservatively, thereby trading off performance in order to maintain a large sensing margin for reliable operation.
In advanced technologies (i.e. 100 nm and smaller gate geometry) RDV is becoming a major bottleneck for improving performance. As device variation increases, timing uncertainty for signal arrival and data capture increases, requiring larger data capture margins, and therefore limiting performance.
Due to its single-ended nature, the ML sensing performed during the CAM search operation is even more sensitive to RDV than the differential sensing used in the SRAM read circuitry. Thus, to maintain reliable operation, most ML sensing schemes employ full-swing sensing which is both slow and power-inefficient.
Self-referenced sense amplifiers address the problems associated with RDV by performing a self-calibration to their respective thresholds to reduce effects of random device variation between adjacent sense amplifiers. However, conventional self-referenced sense amplifiers require a globally timed signal, i.e., a clock-based signal that is applied to plural sense amplifiers, to stop the precharge phase and begin the evaluation phase. Using a globally timed signal causes a plurality of sense amplifiers to have the same amount of precharge time. However, due to process variations, some sense amplifiers may not require the full precharge time in order to reach their particular precharge level. This results in some sense amplifiers sitting idle in the precharge phase after they have reached their precharge level but before the globally timed signal turns off the precharge.